At a Glance

Why Get Tested?

When to Get Tested?

As part of a health checkup or when symptoms suggest hypo- or hyperthyroidism due to a condition affecting the thyroid

Sample Required?

A blood sample drawn from a vein in your arm

Test Preparation Needed?

None needed; however, certain medications can interfere with the tests included in the panel, so tell your doctor about any drugs that you are taking.

The Test Sample

What is being tested?

A thyroid panel is a group of tests that are often ordered together to help evaluate thyroid gland function and to help diagnose thyroid disorders. The tests included in a thyroid panel measure the amount of thyroid hormones in your blood. These hormones are chemical substances that travel through the bloodstream and control or regulate your body's metabolism—how it functions and uses energy.

TSH is produced by the pituitary gland and is part of the body's feedback system to maintain stable amounts of the thyroid hormones T4 and T3 in the blood. When concentrations decrease in the blood, the pituitary is stimulated to release TSH. The TSH in turn stimulates the production and release of T4 and T3 by the thyroid gland. When the system is functioning normally, thyroid production turns on and off to maintain constant blood thyroid hormone levels. The thyroid panel usually includes:

Sometimes a T3 resin uptake (T3RU) test is included to calculate, along with the T4 value, the Free Thyroxine Index (FTI), another method for evaluating thyroid function that corrects for changes in certain proteins that can affect total T4 levels.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm.

How is it used?

The preferred test to screen for thyroid disorders is a TSH test. If your TSH level is abnormal, it will usually be followed up with a test for total T4 or free T4. Sometimes a total T3 or free T3 will also be performed. Often, the laboratory will do this follow-up testing automatically and this is known as reflex testing. This saves your doctor time from having to wait for the results of the initial test and then requesting the additional testing to confirm or clarify a diagnosis. Follow-up tests are often performed on the original sample that was submitted when the initial test was requested. A thyroid panel may be requested by your doctor to have all three tests performed at the same time to get a more complete picture.

What does the test result mean?

If the feedback system involving the thyroid gland is not functioning properly due to one of a variety of disorders, then increased or decreased amounts of thyroid hormones may result. When TSH concentrations are increased, the thyroid will make and release inappropriate amounts of T4 and T3, and the person may experience symptoms associated with hyperthyroidism. If there is decreased production of thyroid hormones, the person may experience symptoms of hypothyroidism.

The following table summarizes test results and their potential meaning.

Is there anything else I should know?

In the past, panels of tests were more common. More recently, however, the practice is to order, where possible, one initial or screening test and then follow up with additional testing, if needed, to reduce the number of unnecessary tests. With thyroid testing, one strategy is to screen with a TSH test and, if the results are abnormal, then order additional tests. Many laboratories still perform thyroid panels, but this will become less common in the future as better screening strategies are developed.

3. What is reverse T3?

Reverse T3 (RT3 or REVT3) is a biologically inactive form of T3. Normally, when T4 is converted to T3 in the body, a certain percentage of the T3 is in the form of RT3. When the body is under stress, such as during a serious illness, thyroid hormone levels may be outside of normal ranges even though there is no thyroid disease present. RT3 may be elevated in non-thyroidal conditions, particularly the stress of illness. It is generally recommended that thyroid testing be avoided in hospitalized patients or deferred until after a person has recovered from an acute illness. Use of the RT3 test remains controversial, and it is not widely requested.

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

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This article was last reviewed on March 3, 2011. | This article was last modified on November 3, 2014.

The review date indicates when the article was last reviewed from beginning to end to ensure that it reflects the most current science. A review may not require any modifications to the article, so the two dates may not always agree.

The modified date indicates that one or more changes were made to the article. Such changes may or may not result from a full review of the article, so the two dates may not always agree.